ACERC
Abstracts - 1990 |
ACERC
Abstracts - 1990 |
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Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
Bonin, M.P. and Queiroz,
M.
The American Society of Mechanical Engineers, New York, HTD-142, 57-66,
Farouk, B. et al., Eds., Heat Transfer in Combustion Systems, 1990. Funded by
ACERC.
A commercially available, laser-based particle analyzer capable of measuring the distribution of size and velocity of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid droplets burning in a turbulent, co-flowing air stream. Previous applications of this instrument have focused on light absorbing particles such as pulverized coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to light droplets larger than 100 µm, including the development of an instrument response function specific to non-absorbing particles in this size class.
A parametric study which investigated the influence of gas-phase turbulence, fuel type, and initial droplet size on the droplet vaporization rate was conducted. Gas phase temperature and velocity measurements were made using thermocouples and hot wire anemometry. Comparisons between measured and predicted droplet size using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Limitations in single stream measurements have been encountered in the lower portion of the flame, relative to the uniform particle flux requirement in the sample volume. Under certain experimental condition, droplet coalescence was also observed downstream of the ignition point.
Queiroz, M. and Yao, S.C.
Accepted for publication in Combustion and Flame, 1990. Funded by ACERC
(National Science Foundation and Associates and Affiliates).
The effect of turbulence intensity and fuel vapor pressure on the thermal structure in a linear array of burning droplet streams has been investigated, using two levels of turbulence intensity and fuel vapor pressure. Particular attention has been focused on the relationship between the dynamic motion of the flame front and the thermal structure of the flame. Bare microthermocouples, digitally compensated for thermal inertia effects, were used to measure fluctuating gas-phase temperatures in this dilute spray flame with 407 mm nominal diameter hexane droplets. The flame was formed by nine vertical streams oriented in a plane and horizontally separated by a distance of 4 mm. Increasing the vapor pressure of the fuel caused higher flame temperatures at the average location of the premixed-gas flame. However, the essential features of the average and fluctuating temperature profiles as well as the pdf surfaces were unchanged over the range of vapor pressures investigated. The higher turbulence intensity level promoted higher temperature fluctuations because the fluctuating combustion zone of the flame was widened. The dynamics of the drifting motion of the flame as well as the instantaneous temperature profiles across the flame were also influenced, causing the disappearance of the bimodal shape in the pdf surfaces.
Queiroz, M.
Accepted for publication in Comb. Sci. & Tech., 1990. Funded by ACERC
(National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 mm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially-premixed flame which acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Bonin, M.P. and Queiroz,
M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
Queiroz, M.
Comb. Sci. & Tech., 72, 1-16, 1990. Funded by ACERC.
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 µm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially premixed flame that acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Montgomery, C.J.; Son, S.F.
and Queiroz, M.
Heat Transfer in Combustion Systems, 142, 1990. Funded by ACERC.
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
Son, S.F.; McMurtry, P.A.
and Queiroz, M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
Rasmussen, K.G. and Queiroz,
M.
The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina,
Brazil, 1990. Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
Clarksean, R. and McMurtry,
P.A.
AIAA Fluid Dynamics, Plasma Dynamics and Lasers Conference, Paper 90-1495,
Seattle, Washington, 1990. Funded by ACERC and Environmental Protection Agency.
A numerical algorithm is presented for studying mixing processes in turbulent flows. The approach is a combination of the spectral method and the compact finite difference technique. The compact method is fourth order accurate in space and has good phase error characteristics. In addition, the compact finite difference technique is easily implemented on variable grids. The fourth order accuracy is only degraded to order 2.4 accuracy on large aspect ratio grids (6:1) for the two-dimensional advection equation. The application of this method is illustrated by performing direct numerical simulations of a spatially developing mixing layer. The evolving flow field is visualized by contour plots of vorticity magnitude and the scalar field. These figures show the shedding and pairing of vortices similar to previously conducted experimental work. The three-dimensional results show secondary structures developing, which enhance the mixing process. The structure of the three-dimensional flow field is similar to that observed experimentally, illustrating the ability of this hybrid scheme to accurately simulate the unsteady development of incompressible flows.
Butler, B.W. and Webb, B.W.
AIAA/ASME Joint Thermophysics Conference, Seattle, WA, 1990. Funded by
ACERC.
This paper reports experimental measurements of local gas temperatures and incident wall radiant heat flux in an 80 MWe pulverized-coal corner-fired boiler. Gas temperatures were measured using a 4 meter long, triply-shielded suction pyrometer and total wall radiation was determined with an ellipsoidal radiometer. The data include detailed wall radiant heat flux measurements around the periphery of the boiler at several different elevations. Local gas temperature profiles were measured at four axial positions in the boiler, with special attention to the near-burner region. Boiler control settings and a coal chemical and particle size analysis are also presented.
Local gas temperatures in the boiler reached maximum of nearly 1800K near the burners and decayed to 1250K at a position just above the boiler nose. The temperature varied in the burner plane from 600K near the wall to 1800K in the center of the flame zone. Wall radiant heat fluxes varied between 500 kW/m² at the burner level to 100 kW/m² near the boiler nose. The radiation transport to the wall was observed to vary substantially around the periphery of the boiler, with largest variation in the near-burner regions.
McMurtry, P.A. and Givi,
P.
AIAA Progress in Engineering Series, Oran, E. and Boris, J., Editors.,
1990 (In press). Funded by ACERC and NASA Lewis Research Center.
The Navier-Stokes equations, along with appropriate conservation equations of energy and chemical species, are generally accepted to provide an "exact" model for most turbulent combustion phenomena of interest. Unfortunately, the complexity of these equations prohibits both their analytic and numerical solutions except under idealized conditions. To provide engineering tools to predict the performance of turbulent combustion systems, it is necessary to resort to approximate analyses in which the effects of turbulence are not treated exactly, but are incorporated by means of turbulence models.
An overview of spectral methods is given, together with a discussion of their implementation in the numerical solution of fluid transport. Various classifications of spectral methods and their convergence properties are described. The "spectral element" method, which constitutes a new category of spectral approximations, is also presented. Its flexibility in dealing with complex flow geometries is highlighted.
A review of the recent applications of spectral methods to reacting flow problems is also given. Emphasis is placed primarily on the interpretation of the physical phenomena captured by spectral simulations of reactive flows. There is some mention of spectral simulations of the turbulent mixing of a passive scalar, but only where an explicit connection to chemical reactions has been made. We conclude with an evaluation for the benefits and limitations of spectral methods, and some speculation of their contributions for turbulent combustion research in the future.
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